Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly/idempotent/non-destructive/closed-world, so the description's job is to add context, and it does: it discloses cost ('Free') and, importantly, that documents are validated in memory and never stored or logged. It also sketches the verdict vocabulary (ACCEPTED | ACCEPTED_WITH_WARNINGS | REJECTED) and finding severities, which helps an agent interpret results; rate limits and error behavior are the only notable omissions.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the core action and derivation rule, then example, then response shape. It loses a little efficiency by enumerating the full response field list (findings[], counts, data_confidence, sources[], disclaimer) that the output schema already provides, but every clause remains readable and relevant.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With an output schema, annotations, and a single fully-described parameter, the definition covers what the tool does, its cost, and its privacy posture. It never says explicitly where the derived Peppol endpoint appears in the response, nor how it relates to validate_invoice, which is the one real gap for an agent deciding between the two.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and the single parameter is documented, so the baseline is 3; the description goes beyond by stating the 15-digit length rule and supplying a formatted sample ('100 1234 5670 0003') that demonstrates the accepted spacing, which the schema only alludes to.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('Format check of a UAE Tax Registration Number') plus a secondary function (deriving the TIN / Peppol participant identifier with scheme 0235). The 'format check' framing implicitly separates it from sibling validate_invoice, which handles whole documents, so an agent can pick between them from the text alone.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives useful calling context ('Free', a concrete example TRN) and the format-only scope, but never states when to prefer this over validate_invoice or the readiness_report path, nor any exclusion such as 'does not verify the TRN against a registry'. Usage is implied rather than directed.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.